23Na imaging may have some advantages over proton imaging for certain diagnostic purposes. In order to use this technique it is important to understand the various factors determining the 23Na relaxation times. It is proposed here to perform a systematic investigation of 23Na relaxation in biological tissues: a. To find the components of cells and body fluids responsible for the 23Na relaxation. b. To characterize normal and pathological tissues by their 23Na NMR relaxation and signal intensities. The systems to be investigated are blood and its components, CSF, cultured cells, whole organs such perfused rat hearts and legs and brain of live rats and finally various tumorous tissues, implanted in mice and samples or human tumors removed in surgical operations. In order to obtain this data some techniques will have to be developed for the measurement of multicomponent relaxation times in heterogeneous systems and in measurements using surface coils. As a result we hope to be able to understand sodium relaxation in biological tissues and to be able to differentiate between various kinds of tissue damages such as edema, ischemia, as well as various tumors and the state of their development.